Communication networks typically operate in accordance with a given standard or specification which sets out what the various elements of the network are permitted to do and how that should be achieved. For example, the standard may define whether the user or more precisely, user equipment is provided with a circuit switched service or a packet switched service or both. The standard may also define the communication protocols which shall be used for the connection. The given standard also defines one or more of the required connection parameters. The connection parameters may relate to various features of the connection. The parameters may define features such as the maximum number of traffic channels, quality of service and so on and/or features that relate to multislot transmission.
In other words, the standard defines the “rules” and parameters on which the communication within the communication system can be based. Examples of the different standards and/or specifications include, without limiting to these, specifications such as GSM (Global System for Mobile communications) or various GSM based systems (such as GPRS: General Packet Radio Service), AMPS (American Mobile Phone System), DAMPS (Digital AMPS), WCDMA (Wideband Code Division Multiple Access) or CDMA in UMTS (Code Division Multiple Access in Universal Mobile Telecommunications System) and so on.
The user equipment i.e. a terminal that is to be used for communication over a particular communication network has to be implemented in accordance with the predefined “rules” of the network. A terminal may also be arranged to be compatible with more than one standard or specification, i. e. the terminal may communicate in accordance with several different types of communication services. These user equipment are often called multi-mode terminals, the basic example thereof being a dual-mode mobile station.
An example of a communication network is a cellular radio network consisting of access areas provided by cells. In most cases the cell can be defined as a certain access area covered by one or several base transceiver stations (BTS) serving user equipment (UE), such as mobile stations (MS), via a radio interface and possibly connected to a base station subsystem (BSS). Several cells cover a larger area, and form typically a radio coverage area referred to as a location area (LA) or in some standards as a routing area (RA). It should be appreciated that the size of the location area or routing area depends on the system and circumstances, and may equal to one cell or be even smaller, such a part of a coverage area of a base station. A feature of access systems such as those provided by the cellular system is that it provides mobility for the mobile stations, i.e. the mobile stations are enabled to move from a location area to another. A mobile station may move even from a network to another network that is compatible with the standard the mobile station is adapted to.
The user equipment (UE) within one of the cells of the cellular system can be controlled by a node providing controller function. Examples of the controller nodes include a base station controller (BSC) and a radio network controller (RNC). In UMTS the radio access network thereof is controlled by a radio network controller (RNC). The controller may control a number of base stations or a base station. The controller can be connected further to a gateway or linking node, for example a gateway GPRS support node (GGSN) or gateway mobile switching center (GMSC), linking the controller nodes to other parts of the communication system and/or to other communication networks, such as to a PSTN (Public Switched Telecommunications Network) or to a data network, such as to a X. 25 based network or to a TCP/IP (Transmission Control Protocol/Internet Protocol) based network. The network may also include nodes for storing information of mobile stations subscribing the networks or visiting the networks, such as appropriate home location registers (HLR) and visitor location registers (VLR).
When a user equipment communicates with a communication network, a communication path has been established between the user equipment and an element or node of the network. The network node is typically one of the controller nodes. At least a part of the communication between the user equipment and the actual destination node will then pass through the controller node. It is possible to transfer i.e. to handover the connection from a first node to a second node. This shall also be possible between two nodes that belong to different network systems. For example, a user equipment having a packet switched (PS) connection with a packet switched network system (e. g. the UMTS) may be handed over to have a circuit switched (CS) connection with a circuit switched network system (e. g. the GSM) and vice versa. The handover of the connection may be required e. g. when the mobile station moves i.e. roams from a cell to another cell. In case the new cell is not served by the same system as the previous cell, the handover needs to be accomplished between different communication systems.
A communication system needs to be able to provide various different functions in order be able to operate. These functions can be divided in different categories. A category comprises functions that relate to the actual carrying of the communication such as voice or multimedia or other data content in the system. Another category can be seen as being formed by control or management functions such as the control of various services and the actual communication. Signaling of messages associated with different functions is thus understood as being implemented on different planes. For example, control messages are communicated on a control plane and the actual communication is then transported on a user plane. The communication on the user plane is supported the signaling of the control messages on the control plane.
Typically the communication systems provide this by means of separate channels, e.g. by means of separated signaling and communication channels. Such arrangements are employed e.g. by signaling system 7 (SS7) core networks and Q.931/GSM/WCDMA subscriber access. Therefore the term “Signaling channel” may be used when referring to control plane communications. Similarly the term communication channel may be used when referring to user plane communications.
The various functions of the communication systems may have been developed quite independently from each other and may use different protocols in different communication systems. The standards and protocols define e.g. which plane shall be used for a certain purpose.
For example, in a third generation (3G) UMTS based communication system, various nodes including a base station, a radio network controller and a serving GPRS support node (SGSN), may be involved in providing user plane communications to a mobile station. A packet-switched intersystem handover from a source network (e.g. a second generation i.e. 2G network) to a target network (e.g. a 3G network) may be performed as defined in Third Generation Partnership Project (3GPP) technical specification TS 43.129, version 6.6.0 (2006-1). This handover scheme assumes the use of SGSN user plane connections in both the source and target network. Thus according to 3GPP TS 43.129, version 6.6.0 protocol data units (PDUs) queued in a serving General Packet Radio Service support node (SGSN) in the source network after a handover is confirmed may be tunneled directly to an SGSN in the target network.
TS 43.129 version 6.6.0 introduces a packet switched handover in order to support real-time packet-switched handover with strict Quality of Service (QoS) requirements on low latency and packet loss. Packet switched handover reduces the service interruption of the user plane information at cell change compared to the cell-reselection and enables methods to improve buffer handling of user plane data in order to reduce packet loss at cell-change.
The scope of TS 43.129 version 6.6.0 is handovers to/from GERAN (GSM/EDGE Radio Access Network) A/Gb mode. More specifically, a serving GPRS support node (SGSN) user plane and control plane is available in both source and target systems. In accordance with this standard lossless packet switched handover PDUs are forwarded from the SGSN in the source system to the SGSN in the target system using a Gn interface.
However, certain access networks may be arranged differently such that they do not employ an SGSN or equivalent entity in the user plane. For instance, in the proposed 3GPP long term evolution (LTE) access scheme, disclosed in 3GPP technical report TR 23.882, version 0.8.1 (2005-11) there are only two user plane elements: an evolved Node B (eNB) and an access Gateway (aGW). 3GPP LTE is a packet-switched only access scheme. When compared to e.g. to the above described examples of third generation systems this means that a radio network controller (RNC) and SGSN user-planes are not used for 3GPP LTE access.
It might therefore be advantageous to have a mechanism which permits intersystem handover to take place in particular in communication systems where the handover may occur between different access systems, such as those described above where one of the networks is a 3GPP LTE access network and one of the networks is a “legacy” 3GPP access network. In this example “legacy” refers to 2G, 2.5G, 3G and 3.5G access networks. It might also be advantageous if packet loss and its associated problems could be avoided during a handover procedure. Embodiments of the present invention aim to address one or more of these problems.